Preparation Bring first 8 ingredients and 3 cups water to a boil in a large saucepan. Reduce heat to medium and simmer until stock yields 1 1/2 cups, about 15 minutes. Set a fine-mesh strainer over a large measuring cup. Strain stock, discarding solids. Cover; keep hot. Set a fine-mesh strainer over another measuring cup. Cut each peeled tomato into 4 wedges. Place wedges, cut side up, on a work surface. Cut away seeds and pulp from tomato and transfer to strainer. Place filleted tomatoes on a double layer of paper towels to drain; sprinkle with 1 teaspoon kosher salt. Pat tomatoes with more paper towels. Let stand for 30 minutes. Press on seeds to yield 1/2 cup tomato juice. Sprinkle gelatin over juice; let stand for 10 minutes to soften. Add to hot stock; whisk vigorously to dissolve gelatin. Stir in 1/4 cup chives, vinegar, and kosher salt to taste. Spray 1 loaf pan with nonstick spray; line with plastic wrap, allowing for a 3" overhang on each side. Smooth plastic to remove wrinkles. Pour 1/2 cup stock into pan. Chill until set, about 40 minutes. Arrange 1 layer of tomatoes in pan, pressing down gently, then drizzle 2 tablespoons stock mixture over. Repeat layering with remaining tomatoes and stock. Pour remaining stock over to fill pan. Cover terrine with plastic wrap. Place on a small rimmed baking sheet. Place second loaf pan on top of terrine. Weigh down terrine by placing 2-3 small canned goods in top pan (some of liquid mixture in bottom pan may spill out). Chill terrine until set, about 6 hours. DO AHEAD: Can be made 2 days ahead. Keep chilled. Uncover terrine; invert onto a platter. Remove pan and plastic wrap. Slice terrine; transfer to plates. Drizzle with oil and sprinkle with chives and sea salt.